Lizzy Caplan flees a marriage proposal from her boyfriend and ends up in the arms of another man, played by Mark Webber (above), in the predictable rom-com “Save the Date.” Photo:

Lizzy Caplan (“Bachelorette”) continues her female-slacker streak as Sarah, a 20-something bookstore manager repelled, not thrilled, by a marriage proposal from her rock-singer boyfriend (Geoffrey Arend), whose bandmate (Martin Starr) happens to be engaged to Sarah’s more conventional sister (Alison Brie). They could all be one big happy family, except Sarah doesn’t want what the other three think she should want.

Claiming to be content on her own, she sidesteps any emotional downtime by running directly into the arms (and bed) of an emo bookstore customer (Mark Webber) with a crush on her. And that’s sort of it, plot-wise. While Caplan works well in theory as an antiromantic-comedy heroine, director and co-screenwriter Michael Mohan just doesn’t give her enough to do.

Mostly, he cuts back and forth between Sarah’s indecision and her sister’s flurry of wedding plans, which predictably generate friction with her fiancé. An elevnth-hour, old-as-the-hills plot twist forces Sarah to come to terms with what she wants and who she wants it with. The more things change . . .